1. Field of the Invention
The present invention relates to a DC-motor in which a DC current supplied from a power supply is commutated by a commutator and is supplied through slip rings into magnetizing coils.
2. Description of the Related Art
Generally, the commutator of a DC-motor is split into multiple segments so as to commutate a current supplied through first brushes from a power supply.
A first type of conventional DC-motors is configured in such a manner that the first brushes are axially in contact with the commutator while second brushes are axially in contact with slip rings that are split into three pieces and circumferentially formed outside the commutator (for example, refer to Patent Document 1).
A second type of conventional DC-motors is configured in such a manner that the first brushes are radially in contact with the commutator while the second brushes are radially in contact with slip rings that are split into three pieces and formed coaxially with the commutator (for example, refer to Patent document 1).
A third type of conventional DC-motors is configured in such a manner that the first brushes are radially in contact with the commutator while the second brushes are axially in contact with slip rings that are split into three pieces and formed outside the circumference of the commutator (for example, refer to Patent Document 2).
Patent Document 1: Japanese Patent Laid-Open No. 2000-230657 (FIG. 1 and FIG. 5)
Patent Document 2: International publication No. 2001-05018 (FIG. 6)
In the first-type DC-motor, because the first brushes slide in contact with the commutator, the first brushes wear away with the lapse of time. Meanwhile, decreasing the density of a current flowing through the first brushes can reduce electrical wear of the first brushes, enhancing their wear resistance. However, in order to achieve that, it is necessary to enlarge areas through which the first brushes are in contact with the commutator. To enlarge the first brushes' contact areas with the commutator, it is also necessary to enlarge the commutator that is in contact with the first brushes, leading to a bigger outer-diameter disk on which the commutator is provided. The bigger the diameter of the disk becomes, the more increased the moment of inertia thereof and the sliding-frictional resistance working between the commutator and the first brushes are; therefore, there has been a problem in that the control-response of the DC-motor is deteriorated.
Patent Document 1 illustrates a first-type DC-motor in which the overall lengths of the first brushes and the second brushes are almost equal. However, in an actual first-type DC-motor, because the first brushes are in contact with the commutator, electrical polarities at the tips of the first brushes change frequently, resulting in their electrical wear; therefore, the first brushes are used that have an overall length longer than that of the second brushes. That is, the first brushes, having an overall length longer than that of the second brushes, are axially in contact with the commutator, resulting in a large axial dimension of the DC-motor; therefore, a problem has been that the DC-motor can not be downsized.
In the second-type DC-motor, because the first brushes are radially in contact with the commutator, the external diameter of the disk on which the commutator is provided does not need to be made larger in order to enlarge contact areas of the first brushes. Therefore, the contact areas can be enlarged while maintaining a good control-response of the DC-motor, so that the wear resistance of the first brushes can be enhanced.
However, the second brushes are radially in contact with the slip rings placed coaxially with the commutator, and the three slip rings are axially placed in a stack, which results in a large axial dimension of the DC-motor; therefore, there has been a shortcoming in that the DC-motor can not be downsized.
In the third-type DC-motor, because the first brushes are radially in contact with the commutator, the contact areas can be enlarged, similarly to the second-type DC-motor, while maintaining a good control-response of the DC-motor, so that the wear resistance of the first brushes can be enhanced as well.
However, because the commutator is annularly placed inside the slip rings, the commutator needs to be provided, protruding outwardly in an axial direction, at the center of the disk in order to be radially in contact with the first brushes, which results in a large axial dimension of the DC-motor; therefore, there has been a shortcoming in that the DC-motor can not be downsized.